Crosstalk in a multi-pair cable is the unwanted coupling of signals from one wire pair to another. When crosstalk is measured at the same end of the cable where the crosstalk originates, the technique is called near end crosstalk (NEXT) measurement. Twisted pair LAN technologies, such as 10BASE-T, 100BASE-T, and Token Ring are primarily vulnerable to cable crosstalk problems that can be tested by measuring the NEXT of the installed cable.
When crosstalk is measured at the end of the cable opposite from where the crosstalk originates, the technique is called far end crosstalk (FEXT) measurement. FEXT is measured by applying a test signal to a wire pair at a far end of the cable and measuring the disturbance on the other wire pairs in the cable at the other or near end. It is relevant to specify the FEXT performance of cabling for network technologies, such as the new 1000BASE-T specification, that transmit simultaneously on multiple wire pairs in the same direction.
While it is easy to measure the FEXT performance of an installed multi-pair cable, it is difficult to specify certification limits for such measurements since FEXT varies with the cable length. The equal level far end crosstalk (ELFEXT) measurement technique was developed as a practical alternative for field certification. Generally, ELFEST equals FEXT minus attenuation caused by the cable. ELFEXT measurements compensate for the effect of varying cable length so that all installed cable can be certified to the same limit.
Residual crosstalk is any signal that is due to the test instrument itself. Residual crosstalk error must be taken into account in crosstalk measurement analysis.
A test signal is generally applied to a wire pair in a differential mode. That is, the signals on the wires of a wire pair are ideally equal in amplitude and opposite in phase (180.degree. out of phase). These conditions reflect an ideal output signal balance (OSB). A differential receiver, receiving a signal with common mode and differential mode components of the wires, will ideally reject the common mode component and respond only to the differential component. This characteristic reflects an ideal common mode rejection (CMR).
However, these conditions are hard to achieve due to imperfect components used to make the test unit and to asymmetries in layout. For example, the output terminals of a test unit can have unbalanced parasitic capacitances. In the past, such imbalance has been minimized by careful selection of components and layout or placement within the test unit assembly. This selection and placement process has been costly and time consuming.
Accordingly there is a need for a test instrument that minimizes deviation of OSB and CMR from the perfect or ideal conditions of equal amplitude and opposite phase. There is also a need for a method to calibrate such an instrument.